A digital signal processing technology is conventionally applied to a wide range of fields such as an image display device like a projector. For example, a histogram conversion method is a technology for correcting an image gradation, and is also useful for improving a contrast of an image projected by a projector. In view of a projector field characteristic, further improvements of the histogram conversion method have been made.
A patent document 1 discloses an example of such technology improvements. Originally, a histogram conversion has been performed with regard to all luminance ranges. However, in the patent document 1, a histogram conversion is performed with regard to a range from a minimum value to a maximum value in an effective video period of a video luminance signal, using a gradation correction coefficient according to a difference between the maximum value and the minimum value.
FIG. 1 is a functional block diagram showing a construction of an image processor of the conventional technology. As shown in FIG. 1, an image processor 20 includes a minimum value detecting circuit 2001, a maximum value detecting circuit 2002, a differential device 2003, a correction coefficient calculating circuit 2004, and a correcting circuit 2005. When receiving an input video luminance signal, the image processor 20 detects a minimum value in an effective video period of the input video luminance signal in the minimum value detecting circuit 2001, and a maximum value in the maximum value detecting circuit 2002. Then, a difference between the maximum value and the minimum value is obtained in the differential device 2003, and a correction coefficient is determined according to the difference in the correction coefficient calculating circuit 2004. The input video luminance signal is corrected according to the correction coefficient in the correcting circuit 2005, and the corrected input video luminance signal is outputted as an output video luminance signal. This further improves an image contrast.    Patent Document 1: Japanese Published Patent Application No. 3549356